This invention relates to electrochemical batteries.
There is a need for cost-effective batteries having high energy densities and moderate-to-high electrical discharge rates. Batteries of this type, such as mechanically rechargeable primary batteries, secondary batteries, and fuel cells, can be used to power devices ranging from flashlights to electrically powered vehicles.
Non-aqueous batteries may have potential as current and voltage sources if challenges relating to environmental compatibility, conductivity, cost, safety, and power density can be overcome. In particular, a variety of metal/molten-sulfur batteries have been developed, with a primary focus on sodium/sulfur cells. The light weight of sulfur makes these systems attractive for electrochemical energy storage. However, those cells may operate at temperatures of between 300.degree. to 350.degree. C. in order to maintain the sodium in a liquid state and to obtain adequate electrolyte conductivity (D. Linden, Handbook of Batteries, McGraw-Hill, NY (1984)). Such temperatures can raise material constraints; corrosion, thermal cycling and cell fabrication raise additional challenges.
An alternative low-temperature, water-based battery, described by Licht et al., U.S. Pat. No. 4,828,942, incorporated herein by reference, includes an electrolytic solution containing a high concentration of reducible sulfur; this solution retains high Coulombic efficiencies (i.e., the ability of the anode to generate charge), similar to those of molten-sulfur batteries, yet operates at a moderate temperature and is highly conductive. Unfortunately, while there exists a large energy difference between sulfur and other materials used for second half-cells in the battery, many preferred materials (such as tin) have generally resulted in low battery voltages. (Licht, J. Electrochem. Soc. 134: 2137 (1987)).
In addition, metals such as aluminum and lithium can undergo a rapid chemical reaction in concentrated aqueous alkaline or alkaline polysulfide electrolytes. For example, it is expected that sulfur, when dissolved in solution and in contact with aluminum, will result in a chemical reaction which is highly exothermic and expected to interfere with the electrochemical oxidation of aluminum: EQU Al+yS.sub.x.sup.-2 +yH.sub.2 O.fwdarw. 1/2Al.sub.2 S.sub.3 +yOH.sup.- +yHS.sup.- .DELTA.G.degree.=-230 kJ/mole: (1)
Al can also decompose to Al(OH).sub.3 via an exothermic chemical reaction: EQU Al+1.5S.sub.2-2 +3H.sub.2 O.fwdarw.Al(OH).sub.3 +3HS.sup.- .DELTA.G.degree.=-509 kJ/mole. (2)
These reactions hinder the performance of the electrochemical cell, resulting in the release of heat rather than electricity. Strategies used to avoid undesired reactions, such as maintaining a large ion flux near the anode, or isolating the ions from the anode, are described in PCT publication no. WO94/16468 (published July, 1994), and U.S. Pat. No. 5,413,881, the latter hereby being incorporated by reference. The electrochemical cells described therein contain two half-cells positioned in electrochemical contact with each other; the half-cells contain, respectively, polysulfide and alkaline solutions.